1. Field
The present application relates to linear actuators, and more particularly, to linear actuators used in prosthetic or orthotic joints.
2. Description of the Related Art
Various types of prosthetic devices are available as artificial substitutes for a missing body part, such as an arm or leg. Prosthetic joints are also available as substitutes for human joints, such as an ankle or knee. Prosthetic joints can include linear actuators to create motion of the joint.
Screw type linear actuators include a screw threadingly coupled to a rotating nut or rotor and function by translating rotation of the nut into linear motion of the screw. The interaction of the threads of the screw and nut can generate a friction force that inhibits rotation of the screw and nut relative to each other. Thus, the actuator can be self-locking under certain conditions, particularly when loaded.
A disadvantage of such actuators is the difficulty in balancing a desired locking force with a desired speed of linear displacement during motion. Screws with smaller pitches generally provide better self-locking properties. However, reducing the thread pitch also reduces the speed of linear motion because a smaller pitch results in a smaller axial displacement of the screw per rotation of the nut.